As a catalyst for purifying the NOx contained in exhaust gas when burning fuel under a lean air-fuel ratio, a catalyst comprised of a catalyst carrier on the surface of which a layer of a NOx absorbent comprised of an alkali metal or alkali earth is formed and further carrying a precious metal such as platinum on the carrier is known (see Japanese Patent No. 2600492). In this catalyst, when the air-fuel ratio of the exhaust gas is lean, the NOx contained in the exhaust gas is oxidized by the platinum and absorbed in the NOx absorbent in the form of nitric acid or nitrous acid. Next, if the combustion chamber or exhaust gas is supplied with a reducing agent and the air-fuel ratio of the exhaust gas is made rich in a short time, the NOx absorbed in the NOx absorbent during this time is released and reduced, then if the air-fuel ratio of the exhaust gas is again returned to lean, the action of absorption of NOx into the NOx absorbent is started.
However, the majority of the NOx contained in exhaust gas is nitrogen monoxide NO, therefore with the above-mentioned catalyst, the NO produced in the interval from when the air-fuel ratio of the exhaust gas is made rich to when the air-fuel ratio of the exhaust gas is next made rich, that is, the NO exhausted from the combustion chamber during this interval, is absorbed at the NOx absorbent in the form of nitric acid or nitrous acid. When the reducing agent is supplied and the air-fuel ratio of the exhaust gas is made rich, the nitric acid or nitrous acid in the NOx absorbent is decomposed by the reducing agent and released from the NOx absorbent and reduced. That is, when the air-fuel ratio of the exhaust gas is made rich, an amount of NO commensurate with the reducing agent is released from the NOx absorbent and reduced.
However, in actuality, the ability of the reducing agent to decompose the nitric acid or nitrous acid is not 100 percent, so to reduce the NO absorbed in the NOx absorbent, a greater amount of reducing agent than the amount of reducing agent necessary for reducing the NO absorbed in the NOx absorbent becomes necessary. Therefore, in practice, when using the above-mentioned catalyst, the amount of reducing agent supplied for releasing the NO from the NOx absorbent is made greater than the amount of the reducing agent necessary for reducing the NO flowing into the catalyst in the interval from when the reducing agent is supplied the previous time to when the reducing agent is supplied the current time.
Now, when the engine is operated at a high speed, the combustion temperature rises, so the amount of generation of NOx increases, therefore the concentration of NO in the exhaust gas increases. Further, when the engine is operated at a high speed, the amount of NO which the catalyst can hold is reduced. In this way, when the engine is operated at a high speed, the concentration of NO in the exhaust gas increases and the amount of NO which the catalyst can hold is decreased, so the NOx absorbing ability of the NOx absorbent ends up becoming saturated in a short time. Therefore, when the engine is operated at a high speed under a lean air-fuel ratio, it is necessary to frequently supply the reducing agent so that the NOx absorbing ability of the NOx absorbent does not become saturated.
Therefore, even if burning fuel under a lean air-fuel ratio so as to improve the fuel efficiency, if frequently supplying the reducing agent, the great difference from the fuel efficiency when continuously burning fuel under a stoichiometric air-fuel ratio ends up disappearing. Further, continuously burning fuel under a stoichiometric air-fuel ratio results in better emission, so burning fuel under a lean air-fuel ratio ends up becoming completely meaningless.